1. Field of the Invention
The present invention relates to a heat-sensitive recording material. More specifically, it relates to a heat-sensitive recording material, which has a high color optical density and has excellent image stability, chemical resistance, head matching (sticking) property, suitability for ink jet recording and ink jet sheet resistance.
2. Description of the Related Art
In general, a heat-sensitive recording material is relatively inexpensive, and a recording appliance therefor is compact and maintenance-free. Accordingly, the heat-sensitive recording material has found wide acceptance. Recently, competition in sales of the heat-sensitive recording material has intensified, and a higher performance distinguished from an ordinary performance is being required of the heat-sensitive recording materials. To meet requirement, studies on a color optical density, an image stability and a head matching property of a heat-sensitive recording material have been earnestly conducted.
In ordinary heat-sensitive recording materials, 2,2-bis(4-hydroxyphenyl)propane (so-called xe2x80x9cbisphenol Axe2x80x9d) has been widely used as an electron-accepting compound which, reactions with an electron-donating colorless dye to make a colorless coating material, develop a color. Nevertheless, in these heat-sensitive recording materials, a sensitivity, background fogging, an image stability, a chemical resistance and a heat matching (sticking) property have not all been satisfied at the same time.
Japanese Patent Application Publication (JP-B) No. 4-20792 discloses a recording material which uses an N-substituted sulfamoylphenol or an N-substituted sulfamoylnaphthol as an electron-accepting compound, and discloses that this recording material (pressure-sensitive, heat-sensitive) has improved image density, image stability and cost. However, there is still room for further improvements to the image density and the image stability.
Moreover, when a full-color information is recorded in a heat-sensitive recording material, recording is sometimes conducted using ink jet printing ink. However, when an ordinary heat-sensitive recording material is subjected to ink jet recording, a color of ink is not faithfully reproduced, nor is a vivid color given. A color may become dull, and information recorded in a heat-sensitive mode may be lost. Further, when a sheet that was recorded by an ink jet system comes into contact with a heat-sensitive recording paper, a heat-sensitive recording image may be lost.
An object of the present invention is to provide a heat-sensitive recording material having a high color optical density and an excellent stability of an image area, chemical resistance, ink jet recordability, ink jet sheet resistance and head matching (sticking) property.
The present inventors have assiduously researched electron-accepting compounds, sensitizers and image stabilizers, and have developed an excellent heat-sensitive recording material to complete the present invention.
That is, the object of the present invention is attained by the following ways.
A first aspect of the present invention is a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-developing layer including an electron-donating colorless dye, 4-hydroxybenzenesulfoanilide as an electron-accepting compound, and 2-benzyloxynaphthalene and ethylenebisstearic acid amide as a sensitizer, wherein a mass ratio (x/y) of the 2-benzyloxynaphthalene (x) to the ethylenebisstearic acid amide (y) is from 95/5 to 40/60.
A second aspect of the present invention is a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-developing layer including an electron-donating colorless dye, 4-hydroxybenzenesulfoanilide as an electron-accepting compound, 2-benzyloxynaphthalene and methylolstearic acid amide as a sensitizer, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane as an image stabilizer, wherein a mass ratio (x/y) of the 2-benzyloxynaphthalene (x) to the methylolstearic acid amide (y) is from 95/5 to 40/60.
A third aspect of the present invention is the heat-sensitive recording material, according to the first aspect, wherein the heat-sensitive color-developing layer further comprising, as an image stabilizer, at least one of 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane.
A fourth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein a dry weight coating amount of the electron-donating colorless dye comprises from 0.1 to 1.0 g/m2.
A fifth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the amount according to the image stabilizer comprises from 10 to 100 parts by mass relative to 100 parts by mass according to the electron-donating colorless dye.
A sixth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the amount according to 4-hydroxybenzenesulfonanilide as the electron-accepting compound comprises from 50 to 400% by mass relative to the electron-donating colorless dye.
A seventh aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the total amount of the sensitizer comprises from 75 to 200 parts by mass relative to 100 parts by mass of 4-hydroxybenzenesulfonanilide as the electron-accepting compound.
An eighth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the heat-sensitive color-developing layer further comprising, as an inorganic pigment, at least one selected from the group consisting of precipitated calcium carbonate, calcium hydroxide and amorphous silica.
A ninth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the heat-sensitive color-developing layer further comprising, as a mordant, a compound including at least one cationic group selected from the group consisting of amide groups, imide groups, primary amino groups, secondary amino groups, tertiary amino groups, a primary ammonium salt groups, secondary ammonium salt groups, tertiary ammonium salt groups and quaternary ammonium salt groups.
A tenth aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, wherein the heat-sensitive color-developing layer is formed by coating with a curtain coater and drying.
An eleventh aspect of the present invention is the heat-sensitive recording material, according to the first or second aspect, further comprising a protective layer disposed on the heat-sensitive recording layer.
In the first aspect of the present invention, the heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-developing layer including an electron-donating colorless dye, 4-hydroxybenzenesulfoanilide as an electron-accepting compound, and 2-benzyloxynaphthalene and ethylenebisstearic acid amide as a sensitizer, wherein a mass ratio (x/y) of the 2-benzyloxynaphthalene (x) to the ethylenebisstearic acid amide (y) is from 95/5 to 40/60.
Further, in the second aspect of the present invention, the heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-developing layer including an electron-donating colorless dye, 4-hydroxybenzenesulfoanilide as an electron-accepting compound, 2-benzyloxynaphthalene and methylolstearic acid amide as a sensitizer, and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane as an image stabilizer, wherein a mass ratio (x/y) of the 2-benzyloxynaphthalene (x) to the methylolstearic acid amide (y) is from 95/5 to 40/60.
With respect to the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer and the support are described below.
(Heat-sensitive Color-developing Layer)
In the first aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer disposed on the support comprises at least the electron-donating colorless dye, the electron-accepting compound and the sensitizer. To further increase the image stability, it is preferable that the heat-sensitive color-developing layer of the present invention includes the heat stabilizer. Moreover, the heat-sensitive color-developing layer may include, as required, a pigment, an adhesive, an ultraviolet absorbent and the like.
In the second aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer disposed on the support comprises at least the electron-donating colorless dye, the electron-accepting compound, the sensitizer and the image stabilizer. Moreover, the heat-sensitive recording material of the present invention may contain, as required, a pigment, an adhesive, an ultraviolet absorbent and the like.
 less than Electron-donating Colorless Dye greater than 
The electron-donating colorless dye according to the present invention is essentially a colorless dye, and has a property of color development by donating an electron or accepting a proton from an acid.
The electron-donating colorless dye of the present invention is preferably one selected from 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-(N-ethyl-N-isoamylamino)fluoran and 2-anilino-3-methyl-6-(N-ethyl-N-propylamino)fluoran. These may be used either singly or in combinations of three or more.
A color optical density, an image stability and a chemical resistance can be all the more improved by using at least one selected from 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-(N-ethyl-N-isoamylamino)fluoran and 2-anilino-3-methyl-6-(N-ethyl-N-propylamino)fluoran.
In the electron-donating colorless dye according to the present invention, except the foregoing three fluoran compounds, for example, 3-di(n-butylamino)-6-methyl-7-anilinofluoran, 2-anilino-3-methyl-6-N-ethyl-N-sec-butylaminofluoran, 3-di(n-pentylamino)-6-methyl-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-ethylamino)-6-methyl-7-anilinofluoran, 3-di(n-butylamino)-7-(2-chloroanilino)fluoran, 3-diethylamino-7-(2-chloroanilino)fluoran and 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran may be used. These may be used in combination with the foregoing preferable fluoran compounds.
When the other fluoran compounds are used in combination, the foregoing preferable three fluoran compounds are included in amounts of, preferably at least 50% by mass, more preferably at least 70% by mass relative to the total amount of the all electron-accepting compounds.
The coating amount of the electron-donating colorless dye is preferably 0.1 to 1.0 g/m2 in terms of a dry weight. It is more preferably from 0.2 to 0.5 g/m2 to improve a color optical density and suppress background fogging.
 less than Electron-accepting Compound greater than 
The heat-sensitive recording material of the present invention comprises 4-hydroxybenzenesulfonanilide as the electron-accepting compound. Since the heat-sensitive recording material of the present invention comprises 4-hydroxybenzenesulfonanilide in the heat-sensitive color-developing layer as the electron-accepting compound, it is possible to increase a color optical density, suppress background fogging and improve a chemical resistance and a head matching (sticking) property.
The amount of the electron-accepting compound (4-hydroxybenzenesulfonanilide) is preferably from 50 to 400% by mass, more preferably from 100 to 300% by mass, especially preferably from 150 to 250% by mass relative to the electron-donating colorless dye. When the amount thereof is less than 50% by mass, the foregoing effects are sometimes insufficient. When it exceeds 400% by mass, the effects are unnecessarily saturated, and the background fogging may increase or the chemical resistance may be impaired.
As the electron-accepting compound of the present invention, known electron-accepting compounds other than 4-hydroxybenzenesulfonamilide can also be used in combination unless the effects of the present invention are impaired.
The other known electron-accepting compounds can properly be selected from known compounds. Especially in view of suppressing the background fogging, phenolic compounds or salicylic acid derivatives and polyvalent metal salts thereof are preferable.
Examples of the phenolic compounds include 2,2xe2x80x2-bis(4-hydroxyphenol)propane(bisphenol A), 4-tert-butylphenol, 4-phenylphenol, 4-hydroxydiphenoxide, 1,1xe2x80x2-bis(4-hydroxyphenyl)cyclohexane, 1,1xe2x80x2-bis(3-chloro-4-hydroxyphenyl)cyclohexane, 1,1xe2x80x2-bis(3-chloro-4-hydroxyphenyl)-2-ethylbutane, 4,4xe2x80x2-sec-isooctylidenediphenol, 4,4xe2x80x2-sec-butylilenediphenol, 4-tert-octylphenol, 4-p-methylphenylphenol, 4,4xe2x80x2-methylcyclohexylidenephenol, 4,4xe2x80x2-isopentylidenephenol, 4-hydroxy-4-isopropyloxydiphenylsulfone, benzyl p-hydroxybenzoate, 4,4xe2x80x2-dihydroxydiphenylsulfone, 2,4xe2x80x2-dihydroxydiphenylsulfone, N-(4-hydroxyphenyl)-p-toluenesulfonamide and 2,4-bis(phenylsulfonyl)phenol.
Further, examples of the salicylic acid derivatives and the polyvalent metal salts thereof include 4-pentadecylsalicylic acid, 3,5-di(xcex1-methylbenzyl)salicylic acid, 3,5-di(tert-octyl) salicylic acid, 5-octadecylsalicylic acid, 5-xcex1-(p-xcex1-methylbenzylphenyl)ethylsalicylic acid, 3-xcex1-methylbenzyl-5-tert-octylsalicylic acid, 5-tetradecylsalicylic acid, 4-hexyloxysalicylic acid, 4-cyclohexyloxysalicylic acid, 4-decyloxysalicylic acid, 4-dodecyloxysalicylic acid, 4-pentadecyloxysalicylic acid, 4-octadecyloxysalicylic acid, and zinc, aluminum, calcium, copper and lead salts thereof.
When the other known electron-accepting compounds are used in combination in the present invention, the amounts thereof are preferably at least 50% by mass, and more preferably at least 70% by mass relative to the total amount of all electron-accepting compounds for 4-hydroxybenzenesulfonamilide.
In the present invention, when preparing a coating solution of the heat-sensitive color-developing layer, the particle size of the electron-accepting compound is preferably 1.0 xcexcm or less, and more preferably from 0.4 to 0.7 xcexcm in terms of a volume-average particle size. When the volume-average particle size is 1.0 xcexcm or less, there is an advantage that a heat sensitivity is increased. The volume-average particle size can easily be measured with a laser diffraction-type particle size distribution measuring device (for example, xe2x80x9cLA500xe2x80x9d, manufactured by Horiba Ltd.).
 less than Sensitizer greater than 
In the first aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer comprises 2-benzyloxynaphthalene and ethylenebisstearic acid amide as the sensitizer. Thus, a sensitivity can greatly be improved while suppressing occurrence of background fogging.
In the present invention, the 2-benzyloxynaphthalene (x) to ethylenebisstearic acid amide (y) ratio (x/y) is used in a range of from 95/5 to 40/60. When the mass ratio (x/y) exceeds this range, the sensitivity is decreased. When it is less than the range, the sensitivity is also decreased. The mass ratio (x/y) is preferably from 90/10 to 50/50, and more preferably from 85/15 to 60/40.
The total amount of the sensitizer is preferably from 75 to 200 parts by mass, and more preferably from 100 to 150 parts by mass relative to 100 parts by mass of 4-hydroxybenzenesulfonanilide as the electron-accepting compound. When the total amount of the sensitizer is in the range of from 75 to 200 parts by weight, the effect of improving a sensitivity is great, and the image stability is also good.
The heat-sensitive color-developing layer in the first aspect of the heat-sensitive recording material in the present invention may comprises, in addition to the 2-benzyloxynaphthalene and the ethylenebisstearic acid amide, other known sensitizers that do not impair the effects of the present invention. When the heat-sensitive color-developing layer comprises the other sensitizers, amounts thereof are preferably at least 50% by mass, and more preferably at least 70% by mass relative to the total amount of all sensitizers.
Examples of the other known sensitizers include aliphatic monoamides, stearylurea, p-benzylbiphenyl, di(2-methylphenoxy)ethane, di(2-methoxyphenoxy)ethane, xcex2-naphthol-(p-methylbenzyl) ether, xcex1-naphthylbenzyl ether, 1,4-butanediol-p-methylphenyl ether, 1,4-butanediol-p-isopropylphenyl ether, 1,4-butanediol-p-tert-octylphenyl ether, 1-phenoxy-2-(4-ethylphenoxy)ethane, 1-phenoxy-2-(chlorophenoxy)ethane, 1,4-butanediolphenyl ether, diethylene glycol bis(4-methoxyphenyl) ether, m-terphenyl, oxalic acid methylbenzyl ether, 1,2-diphenoxymethylbenzene, 1,2-bis(3-methylphenoxy)ethane and 1,4-bis(phenoxymethyl)benzene.
Next, in the second aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer comprises 2-benzyloxynaphthalene and methylolstearic acid amide as the sensitizer. Thus, a sensitivity can greatly be improved while suppressing occurrence of background fogging.
In the present invention, the 2-benzyloxynaphthalene (x) to methylolstearic acid amide (y) ratio (x/y) is used in the range of from 95/5 to 40/60. When the mass ratio (x/y) exceeds this range, the sensitivity is decreased. When it is less than the range, the sensitivity is also decreased. The mass ratio (x/y) is preferably from 90/10 to 50/50, and more preferably from 85/15 to 60/40.
The total amount of the sensitizer is preferably from 75 to 200 parts by mass, and more preferably from 100 to 150 parts by mass relative to 100 parts by mass of 4-hydroxybenzenesulfonanilide as the electron-accepting compound. When the total amount of the sensitizer is in the range of from 75 to 200 parts by mass, the effect of improving the sensitivity is increased, and the image stability is also good.
The heat-sensitive color-developing layer of the heat-sensitive recording material in the present invention may comprise, in addition to 2-benzyloxynaphthalene and methylolstearic acid amide, other known sensitizers unless the effects of the present invention are impaired. When the other sensitizers are contained, the amounts thereof are preferably at least 50% by mass, more preferably at least 70% by mass relative to the total amount of all sensitizers.
Examples of the other known sensitizers include aliphatic monoamides, stearylurea, p-benzylbiphenyl, di(2-methylphenoxy)ethane, di(2-methoxyphenoxy)ethane, xcex2-naphthol-(p-methylbenzyl) ether, xcex1-naphthylbenzyl ether, 1,4-butanediol-p-methylphenyl ether, 1,4-butanediol-p-isopropylphenyl ether, 1,4-butanediol-p-tert-octylphenyl ether, 1-phenoxy-2-(4-ethylphenoxy)ethane, 1-phenoxy-2-(chlorophenoxy)ethane, 1,4-butanediolphenyl ether, diethylene glycol bis(4-methoxyphenyl)ether, m-terphenyl, oxalic acid methylbenzyl ether, 1,2-diphenoxymethylbenzene, 1,2-bis(3-methylphenoxy)ethane and 1,4-bis(phenoxymethyl)benzene.
 less than Image Stabilizer greater than 
In the first aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer can include the image stabilizer for improving more a stability of an image area. As the image stabilizer, phenol compounds, especially hindered phenol compounds are effective. Examples thereof include 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-ethyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(3,5-di-tert-butyl-4-hydroxyphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)propane, 2,2xe2x80x2-methylenebis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-methylenebis(6-tert-butyl-4-ethylphenol), 4,4xe2x80x2-butylidenebis(6-tert-butyl-3-methylphenol) and 4,4xe2x80x2-thiobis(3-methyl-6-tert-butylphenol). In the present invention, it is preferable that the heat-sensitive color-developing layer comprises, among these compounds, at least one of 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane.
When the heat-sensitive color-developing layer comprises 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane as the image stabilizer, it is possible to improve the background fogging and greatly improve the stability of the image area. 1,1,3-Tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane (xcex1) and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane (xcex2) may be used either singly or in combination. When they are used in combination, the mass ratio (xcex1/xcex2) is preferably from 20/80 to 80/20, and more preferably from 40/60 to 60/40.
The total amount of the image stabilizer is preferably from 10 to 100 parts by mass, and more preferably from 20 to 60 parts by mass relative to 100 parts by mass of the electron-donating colorless dye in view of efficiently exhibiting desired effects of the background fogging, the image stability, the chemical resistance and the head matching (sticking) property.
Further, in addition to the 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane or the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, other known image stabilizers may be used in combination. When other known image stabilizers are used in combination, the amount of the 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane is preferably at least 50% by mass, and more preferably at least 70% by mass relative to the total amount of all image stabilizers.
Next, in the second aspect of the heat-sensitive recording material in the present invention, the heat-sensitive color-developing layer comprises at least one of 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane as the image stabilizer. When the heat-sensitive color-developing layer comprises the image stabilizer, it is possible to suppress the background fogging and greatly improve a stability of the image area due to a synergistic effect of 4-hydroxybenzenesulfonanilide as the electron-accepting compound and 2-benzyloxynaphthalene and methylolstearic acid amide as the sensitizer.
The 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane (xcex1) and the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane (xcex2) may be used either singly or in combination. When they are used in combination, the mass ratio (xcex1/xcex2) is preferably from 20/80 to 80/20, and more preferably from 40/60 to 60/40.
The total amount of the image stabilizer is preferably from 10 to 100 parts by mass, and more preferably from 20 to 60 parts by mass relative to 100 parts by mass of the electron-donating colorless dye in view of efficiently exhibiting desired effects of the background fogging, the image stability, the chemical resistance and the head matching (sticking) property.
In the second aspect of the heat-sensitive recording material in the present invention, in addition to the 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, other known image stabilizers may be used in combination. When other known image stabilizers are used in combination, the amount of the 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or the 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane is preferably at least 50% by mass, more preferably at least 70% by mass relative to the total amount of all image stabilizers.
As other known image stabilizers, phenol compounds, especially hindered phenol compounds are effective. Examples thereof include 1,1,3-tris(2-ethyl-4-hydroxy-5-cyclohexylphenyl) butane, 1,1,3-tris(3,5-di-tert-butyl-4-hydroxyphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)propane, 2,2xe2x80x2-methylenebis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-methylenebis(6-tert-butyl-4-ethylphenol), 4,4xe2x80x2-butylidenebis(6-tert-butyl-3-methylphenol) and 4,4xe2x80x2-thiobis(3-methyl-6-tert-butylphenol).
 less than Inorganic Pigment greater than 
The heat-sensitive recording material of the present invention may comprise, as required, an inorganic pigment in the heat-sensitive color-developing unless this would impair the effects of the present invention. As the inorganic pigment, it is preferable to use precipitated calcium carbonate, aluminum hydroxide and amorphous silica either singly or in combination. When the precipitated calcium carbonate and/or the aluminum hydroxide and/or the amorphous silica is contained as the inorganic pigment, background fogging or an abrasion of a thermal head can be reduced. It is further possible to prevent adhesion of refuse to the thermal head or improve the sticking property. Moreover, incorporation of amorphous silica can prevent blotting by ink jet recording.
The amount of the inorganic pigment is preferably from 50 to 250 parts by mass, more preferably from 70 to 170 parts by mass, and especially preferably from 90 to 140 parts by mass relative to 100 parts by mass of the electron-accepting compound in view of the color optical density and adhesion of refuse to a thermal head.
Further, a particle size of the inorganic pigment is preferably from 0.6 to 2.5 xcexcm, more preferably from 0.8 to 2.0 xcexcm, and especially preferably from 1.0 to 1.6 xcexcm in terms of a volume-average particle size in view of a color optical density and adhesion of refuse to the thermal head as well.
Precipitated calcium carbonate generally has a crystal form of calcite, aragonite or vaterite. As the inorganic pigment used in the present invention, a precipitated calcium carbonate having a calcite crystal form is preferable in view of absorption and hardness. Further, a particulate form is preferably a spindle-shaped or scalenohedral form.
The precipitated calcium carbonate having a calcite crystal form can be formed by a known method.
Examples of the other inorganic pigment include calcium carbonate except for precipitated calcium carbonate having the calcite crystal form, barium sulfate, lithopone, agalmatolite, kaolin and calcined kaolin. When the precipitated calcium carbonate and an other inorganic pigment are used in combination, the ratio of the total mass (v) of precipitated calcium carbonate and the total mass (w) of the other inorganic pigment (v/w) is preferably from 100/0 to 60/40, more preferably from 100/0 to 80/20.
 less than Adhesive greater than 
In the heat-sensitive recording material of the present invention, the heat-sensitive color-developing layer may contain an adhesive so long as this does not impair the effects of the present invention.
In recent years, as the heat-sensitive recording material has been used in various fields, it has been sometimes employed in offset printing. However, the heat-sensitive recording material generally has a low surface strength, and therefore, has a problem of a poor printability, for example, picking, which may occur during offset printing. In order to solve this problem, the printability in offset printing can be improved by incorporating the adhesive in the heat-sensitive color-developing layer.
As the adhesive, polyvinyl alcohol having a degree of saponification of from 85 to 99 mol % and a degree of polymerization of from 200 to 2,000 is preferable. Incorporation of this polyvinyl alcohol makes it possible to increase an interlaminar adhesion between the heat-sensitive color-developing layer and the support while maintaining a color optical density of the heat-sensitive recording material to improve printability and prevent the picking, for example. The degree of saponification is preferably from 85 to 99 mol %. When the degree of saponification is within the range of from 85 to 99 mol %, it is possible to prevent the picking that takes place due to a poor resistance to wetting water used in offset printing and to prevent, during preparation of a coating solution, formation of an undissolved substance that causes occurrence of a defective portion. Further, it is advisable to use polyvinyl alcohol having a degree of polymerization of from 200 to 2,000. When the degree of polymerization is within the range of from 200 to 2,000, there is no need to increase the amount of the polyvinyl alcohol. Accordingly, the decrease in the image density owing to the increase in the amount does not occur. Moreover, since polyvinyl alcohol is easily soluble in a solvent without increasing a viscosity of a coating solution, its formation and coating thereof are easy. The degree of polymerization herein refers to an average degree of polymerization measured by the method according to JIS-K 6726 (1994).
The amount of polyvinyl alcohol in the heat-sensitive color-developing layer is preferably from 30 to 300% by mass, more preferably from 70 to 200% by mass, especially preferably from 100 to 170% by mass relative to 100 parts by mass of the electron-donating colorless dye in view of a color optical density and an offset printability (picking). The polyvinyl alcohol incorporated in the heat-sensitive color-developing layer acts not only as the adhesive for enhancing an interlaminar adhesion but also as a dispersing agent, a binder or the like.
Polyvinyl alcohol that meets conditions of the degree of saponification of from 85 to 99 mol % and the degree of polymerization of from 200 to 2,000 is preferable. However, in view of the color optical density in recording with the thermal head, it is advisable to use at least one selected from sulfo-modified polyvinyl alcohols, diacetone-modified polyvinyl alcohols and acetoacetyl-modified polyvinyl alcohols.
The sulfo-modified polyvinyl alcohols, the diacetone-modified polyvinyl alcohols and the acetoacetyl-modified polyvinyl alcohols may be used either singly or in combination, or together with another polyvinyl alcohol. When another polyvinyl alcohol is used together, the sulfo-modified polyvinyl alcohol, the diacetone-modified polyvinyl alcohol and the acetoacetyl-modified polyvinyl alcohol are contained in amounts of, preferably at least 10% by mass, and more preferably at least 20% by mass relative to the total amount of polyvinyl alcohols.
The sulfo-modified polyvinyl alcohol can be formed by a method of saponifying a polymer obtained by polymerizing an olefin sulfonic acid such as ethylenesulfonic acid, allylsulfonic acids or metaallylsulfonic acids or salts thereof and a vinyl ester such as a vinyl acetate in alcohol or an alcohol/water mixed solvent, a method of copolymerizing a mid-sodium salt and a vinyl ester such as vinyl acetate and saponifying the resulting copolymer, a method of treating polyvinyl alcohol with bromine or iodine and heating the product in an acidic sodium sulfite aqueous solution, a method of heating polyvinyl alcohol in a high concentration sulfuric acid aqueous solution or a method of acetalizing polyvinyl alcohol with an aldehyde compound containing a sulfonic acid group.
The diacetone-modified polyvinyl alcohol is a partially or completely saponified product of a copolymer of a monomer containing a diacetone group and a vinyl ester, and it is formed by saponifying a resin obtained by copolymerizing a monomer having a diacetone group with a vinyl ester.
In the diacetone-modified polyvinyl alcohol, the content of the monomer (recurring unit structure) having the diacetone group is not particularly limited.
The acetoacetyl-modified polyvinyl alcohol can generally be formed by adding a liquid or gaseous diketene to a polyvinyl alcohol resin solution, dispersion or powder. A degree of acetylation of the acetoacetyl-modified polyvinyl alcohol can be properly selected depending on desired qualities of the heat-sensitive recording material.
 less than Ultraviolet Absorbent greater than 
In the heat-sensitive recording material of the present invention, the heat-sensitive color-developing layer may contain an ultraviolet absorbent unless this would impair the effects of the present invention. Examples of the ultraviolet absorbent that can be used in the present invention are as follows. 
 less than Binder greater than 
In the present invention, the electron-donating colorless dye, the electron-accepting compound and the sensitizer can be dispersed in a water-soluble binder. The water-soluble binder used in this case is preferably a compound, which is dissolved in water of 25xc2x0 C. in an amount of at least 5% by mass.
Specific examples of the water-soluble binder include polyvinyl alcohol, methylcellulose, carboxymethylcellulose, starches (including modified starches), gelatins, gum arabic, casein and a saponified product of a styrene-maleic anhydride copolymer.
These binders are used not only in dispersion but also for improving a film strength of the heat-sensitive color-developing layer. For this purpose, a synthetic polymer latex-type binder such as a styrene-butadiene copolymer, a vinyl acetate copolymer, an acrylonitrile-butadiene copolymer, a methyl acrylate-butadiene copolymer or a polyvinylidene chloride can also be used in combination.
The electron-donating colorless dye, the electron-accepting compound and the sensitizer are dispersed with a stirring pulverizer such as a ball mill, an attritor or a sand mill either simultaneously or separately to form a coating solution. The coating solution may further contain, as required, a pigment, a metallic soap, a wax and a surfactant, as well as an antioxidant, the foregoing ultraviolet absorbent, a defoamer and a fluorescent dye.
As the pigment, calcium carbonate, barium sulfate, lithopone, agalmatolite, kaolin, calcined kaolin, amorphous silica and aluminum hydroxide are used. As the metallic soap, higher fatty acid metal salts are used, examples thereof being zinc stearate, calcium stearate and aluminum stearate.
As the wax, a paraffin wax, a microcrystalline wax, a carnauba wax, a methylolstearoamide wax, a polyethylene wax, a polystyrene wax and a fatty acid amide wax can be used either singly or in combination. As the surfactant, a sulfosuccinic acid alkali metal salt and a fluorine-containing surfactant are used.
Moreover, in the present invention, blotting in ink jet recording can be prevented by incorporating a mordant.
As the mordant, a compound containing at least one cationic group selected from an amide group, an imide group, a primary amino group, a secondary amino group, a tertiary amino group, a primary ammonium salt group, a secondary ammonium salt group, a tertiary ammonium salt group and a quaternary ammonium salt group is preferable. Specific examples thereof can include polyvinylbenzyltrimethylammonium chloride, polydiallyldimethylammonium chloride, polymethacryloyloxyethyl-xcex2-hydroxyethyldimethylammonium chloride, polydimethylaminoethyl methacrylate hydrochloride, polyethylenimine, polyallylamine, polyallylamine hydrochloride, a polyamide-polyamine resin, cationized starch, a dicyandiamide formalin condensate and a dimethyl-2-hydroxypropylammonium salt polymer. The molecular weight of these compounds is preferably from 1,000 to 20,000. When the molecular weight is less than 1,000, a water resistance thereof tends to be unsatisfactory. When it is more than 20,000, a viscosity is increased, and a handleability may become poor.
The components contained in the heat-sensitive color-developing layer are mixed, and then coated on the support. The coating is conducted by a method using an air knife coater, a roll coater, a blade coater or a curtain coater. The heat-sensitive recording material of the present invention is coated on the support, dried, smoothed with a calender, and then actually used. As the coating method, a method using a curtain coater is especially preferable in the present invention because the heat-sensitive color-developing layer can uniformly be coated and a sensitivity and an image stability are efficiently improved.
A coating amount of the heat-sensitive color-developing layer is not particularly limited. It is usually from 2 to 7 g/m2 in terms of a dry weight.
(Support)
As the support used in the present invention, a known support can be used. Specific examples thereof include a paper support such as a woodfree paper, a coated paper obtained by coating a resin or a pigment on paper, a resin-laminated paper, an undercoat base paper having an undercoat layer, a synthetic paper and a plastic film. In view of a thermal head matching property, an undercoat base paper having an undercoat layer is preferable, and an undercoat base paper in which an undercoat layer containing an oil-absorbable pigment is formed with a blade coater is especially preferable.
As the support, a smooth support having a smoothness of at least 300 seconds as defined by JIS-P 8119 is preferable in view of dot reproducibility.
As stated above, the support used in the present invention preferably has the undercoat layer. As the undercoat layer, a layer containing a pigment and a binder as main components is preferable.
As the pigment, all general inorganic and organic pigments are available. Especially, an oil-absorbable pigment having an oil absorption of at least 40 ml/100 g (cc/100 g) as defined by JIS-K 5101 is preferable. Examples of the oil-absorbable pigment can include calcined kaolin, aluminum oxide, magnesium carbonate, calcium carbonate, amorphous silica, calcined diatomaceous earth, aluminum silicate, magnesium aluminosilicate and aluminum oxide. Of these, calcined kaolin having an oil absorption of from 70 to 80 ml/100 g as defined by JIS-K 5101 is especially preferable.
The binder used in the undercoat layer includes water-soluble polymers and aqueous binders. These may be used either singly or in combination.
Examples of the water-soluble polymers include starch, polyvinyl alcohol, polyacrylamide, carboxymethyl alcohol, methylcellulose and casein.
The aqueous binders are generally synthetic rubber latexes and synthetic resin emulsions. Specific examples thereof include a styrene-butadiene rubber latex (SBR), an acrylonitrile-butadiene rubber latex, a methyl acrylate-butadiene rubber latex and a vinyl acetate emulsion.
The amounts of these binders are determined in consideration of a film strength of the coating layer and a heat sensitivity of the heat-sensitive color-developing layer. The amount of the binders are from 3 to 100% by mass, preferably from 5 to 50% by mass, especially preferably from 8 to 15% by mass relative to the pigment added to the undercoat layer. Further, the undercoat layer may contain a wax, an erasing inhibitor and a surfactant.
The undercoat layer can be coated by a known coating method. Specifically, a method using an air knife coater, a roll coater, a blade coater, a gravure coater or a curtain coater is available. Among others, a blade coating method using a blade coater is preferable. Further, smooth treatment using a calender may be applied as required.
The method using the blade coater includes not only a coating method using a bevel-type or bent-type blade but also a coating method using a rod blade or a bill blade. Further, coating may be conducted by not only an off-machine coater but also an on-machine coater installed on a paper machine. For imparting a fluidity in blade coating to obtain an excellent smoothness and an excellent surface condition, carboxymethylcellulose having a degree of etherification of from 0.6 to 0.8 and a weight average molecular weight of from 20,000 to 200,000 may be added to the coating solution for the undercoat layer in an amount of from 1 to 5% by mass, and preferably from 1 to 3% by mass relative to the pigment.
The coating amount of the undercoat layer is not particularly limited but in view of the properties of the heat-sensitive recording material, this coating amount is at least 2 g/m2, preferably at least 4 g/m2, and especially preferably from 7 g/m2 to 12 g/m2.
(Protective Layer)
The protective layer can be disposed, as required, on the heat-sensitive color-developing layer. The formation of the protective layer can improve the image stability and the chemical resistance. The protective layer can contain a binder, a surfactant, an organic or inorganic pigment and a heat-meltable substance.
Examples of the binder include polyvinyl alcohol, modified polyvinyl alcohol, starch, modified starches such as oxidized starch and urea phosphate starch, carboxyl group-containing polymers such as a styrene-maleic anhydride copolymer, a styrene-maleic anhydride copolymer alkyl ester and a styrene-acrylic acid copolymer, a vinyl acetate-acrylamide copolymer, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, gelatins, gum arabic, casein, polyacrylamide derivatives, polyvinyl pyrrolidone, a styrene-butadiene rubber latex, an acrylonitrile-butadiene rubber latex, a methyl acrylate-butadiene rubber latex and a vinyl acetate emulsion. Of these, water-soluble polymers are preferable.
As the water-soluble polymers, polyvinyl alcohol, oxidized starch and urea phosphate starch are preferable. A mixture of the polyvinyl alcohol (v) and the oxidized starch and/or the urea phosphate starch (w) at a mass ratio (v/w) of from 90/10 to 10/90 is more preferable. In case of a combination of the oxidized starch and the urea phosphate starch, namely, a combination of polyvinyl alcohol, oxidized starch and urea phosphate starch, it is preferable to use oxidized starch (W1) and urea phosphate starch (W2) at a mass ratio (W1/W2) of from 10/90 to 90/10.
As the modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, silicon-modified polyvinyl alcohol and amide-modified polyvinyl alcohol are preferably used. Further, sulfo-modified polyvinyl alcohol and carboxy-modified polyvinyl alcohol can be used. When these polyvinyl alcohols are used in combination with a crosslinking agent reactive therewith, better results can be obtained.
The content of the water-soluble polymer is preferably from 10 to 90% by mass, and more preferably from 30 to 70% by mass relative to the solid content of the coating solution for the protective layer.
Preferable examples of the crosslinking agent include polyvalent amine compounds such as ethylenediamine, polyvalent aldehyde compounds such as glyoxal, glutaraldehyde and dialdehyde, dihydrazide compounds such as adipic acid dihydrazide and phthalic acid dihydrazide, water-soluble methylol compounds (urea, melamine and phenol), polyfunctional epoxy compounds and polyvalent metal salts (Al, Ti, Zr and Mg). When the crosslinking agent is used in combination with polyvinyl alcohol, the amount of the crosslinking agent is preferably from 2 to 30% by mass, more preferably from 5 to 20% by mass relative to polyvinyl alcohol. The use of the crosslinking agent can improve the film strength and the water resistance. As the crosslinking agent used in the present invention, polyvalent aldehyde compounds and dihydrazide compounds are preferable.
As the inorganic pigment, for example, aluminum hydroxide and kaolin are preferable. In view of the color optical density by recording with the thermal head, aluminum hydroxide having an average particle size of from 0.5 to 0.9 xcexcm is preferable. Further, as the inorganic pigment, calcium carbonate, zinc oxide, aluminum oxide, titanium dioxide, silicon dioxide, barium sulfate, zinc sulfate, talc, clay, calcined clay and colloidal silica are available. As the organic pigment, a urea-formalin resin, a styrene-methacrylic acid copolymer and polystyrene are available.
The amount of the inorganic pigment is preferably from 10 to 90% by mass, and more preferably from 30 to 70% by mass relative to the solid content of the coating solution for the protective layer.
When the protective layer comprises the inorganic pigment and the water-soluble polymer, the mixing ratio thereof varies with the type and the particle size of the inorganic pigment and the type of the water-soluble polymer. The amount of the water-soluble polymer is preferably from 50 to 400% by mass, more preferably from 100 to 250% by mass relative to the inorganic pigment. The total amount of the inorganic pigment and the water-soluble polymer contained in the protective layer is preferably at least 50% by mass of the protective layer.
In the present invention, the addition of the surfactant to the coating solution for the protective layer can further improve the chemical resistance. Preferable examples of the surfactant include alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, alkylsulfosuccinates such as sodium dioctylsulfosuccinate, polyoxyethylenealkyl ether phosphates, sodium hexametaphosphate and perfluoroalkyl carboxylates. Of these, alkylsulfosuccinates are preferable. The amount of the surfactant is preferably from 0.1 to 5% by mass, and more preferably from 0.5 to 3% by mass relative to the solid content of the coating solution for the protective layer.
The coating solution for the protective layer can contain a lubricant, a defoamer, a fluorescent brightener and an organic color pigment unless this would impair the effects of the present invention are impaired. Examples of the lubricant include metallic soaps such as zinc stearate and calcium stearate, and waxes such as a paraffin wax, a microcrystalline wax, a carnauba wax and a synthetic polymer wax.